Tidmarsh Sensor Expansion Box
A fully uniform sensor network is not practical, both due to the expense of sensors and the varying utility of different sensors in different locations. As the project evolves, we are also adding and removing sensors to best capture the transformation.
To facilitate the addition of sensors to the Tidmarsh sensor node platform, we have developed an expansion box that allows additional probes to be easily connected. The box connects to the main sensor node via the node's I²C expansion connector, and provides four channels of analog I/O and an interface for one-wire temperature sensors.
The expansion box provides an interface similar to other scientific dataloggers, enabling standard probes to be connected to the real-time sensor network.
The board provides four channels of 12-bit analog to digital conversion. These channels accept an 0-3.3 volt range. The fourth channel may optionally be populated as a differential input with a dedicated op amp front end. All channels are buffered by a high-impedance amplifier and antialiasing filter after the multiplexer. The associated op amps are switched off to save power except when a measurement is in progress.
Sensors are powered by programmable excitation voltages, which can be configured between 0.8 and 3.0 volts with 8 bits of resolution. Two power channels are available; analog inputs 1-2 and 3-4 each share power supplies. The power to the sensors can be turned completely off in between samples.
An ATmega168PA microcontroller provides control over the power supplies via the main I²C bus. The microcontroller also provides an interface for one-wire temperature sensors, which are readily and quite cheaply available in waterproof assemblies. Two ports are provided on the board, but the one-wire bus allows many sensors to be connected in parallel.
The expansion board installs into the same type of enclosure as the sensor node itself. M12-size tapped holes accept waterproof cable glands for weather-sealed wire entry, and the bare wires from the sensors go into spring-loaded terminals on the board for rapid installation in the field. Unused openings can be closed with threaded plugs to keep moisture out.
The configuration for connected sensors is stored in the EEPROM on the expansion board. The configured expansion box can then be connected to any sensor nodes and begin sending data without reconfiguring the sensor node itself.